Coagulation and sedimentation of water to be treated have been adopted as a prior treatment of sand filtration. In the present coagulation sedimentation process, an inorganic coagulant is injected into water to be treated to agglomerate fine suspended particles contained in the water to be treated into a flock having a diameter that can be settled and separated, thus conducting sedimentation and separation of the flock by the action of gravity.
A coagulation sedimentation plant is roughly categorized into a conventional system and a rapid coagulation sedimentation basin system, and the latter is further categorized into a slurry circulating system and a sludge blanket system. There is, however, no difference between these systems in that they are involved in sedimentation and separation through a micro flocculation step of suspended particles and a flocculation step of micro flocks.
Then, the search for types and amounts of coagulants and coagulant aids necessary for forming a flock having a greater diameter has been a major technical challenge in a conventional coagulation sedimentation process.
An existing coagulation sedimentation process has been based on the following general equation proposed by Smoluchowski.dN/dt=αβninj  [Equation 1]
Wherein N stands for the number of fine suspended particles and micro flocks in unit volume; α, collision efficiency which stands for deposit efficiency when two particles collide, and will vary depending on an inorganic coagulant; β, collision frequency of two particles; ni, the number of particles which will flow per unit volume; and nj, the number of existing particles in unit volume.
In addition, dN/dt expressed by the above general equation indicates a speed of reducing fine suspended particles and micro flocks per unit time, which is referred to as flock forming speed.
Incidentally, a coagulation theory based on the above described Smoluchowski equation has been explained in such a way that, for example, as disclosed in Non-Patent Document 1, a conventional process is used to divide a step of agglomeration into two steps, more specifically a micro flocculation step for neutralizing the charge of fine suspended particles contained in water to be treated and agglomerating these suspended particles into micro flocks whose diameter is approximately 3.0 μm, is dependent on Brownian motion, and a flocculation step for agglomerating the micro flocks whose diameter is 3.0 μm or greater into flocks which can be settled and separated is dependent on whether or not agitation is conducted by force of agitation greater than a predetermined level.
However, on the other hand, Non-Patent Document 2 has reported that flocks will be destroyed on agitation conducted strongly and rapidly. Further, due to influences of an explanation that the flocks are destroyed by shearing force which deteriorates the flock surface, slow agitation relatively lower in agitation intensity has been adopted in a flock forming step.
In reality, rapid coagulation sedimentation basins have been mostly developed in the U.S.A. However, as described above, agitation by water streams lower in agitation intensity has been adopted in a micro flocculation step due to the influence of Non-Patent Document 2.
On the other hand, as shown in Non-Patent Document 3, the Smoluchowski equation has indicated that an increase in collision frequency β, that is, an increase in agitation intensity, is effective in agglomeration. An experiment has been done by using, for example, a sludge blanket-type rapid coagulation sedimentation basin to increase the agitation intensity rapidly. However, a conclusion was reached in the above experiment that where a strong agitation is kept for a prolonged period of time in a micro flocculation step, that is, where a GR value which is rapid agitation intensity and a TR value which is rapid agitation time are increased, a original flock is destroyed to increase the turbidity of sedimentation-treated water, which is the same as that reported in conventional experiments. As a result, the above-described rapid agitation system is rarely adopted.
As described above, in response to a request so far made for improving the quality of filtered water, on coagulation and sedimentation which are prior treatments, an operation heavily depending on an increased injection rate of inorganic coagulants on the precondition to accelerate the agglomeration of suspended particles and suppress the destruction of flocks has been adopted. In particular, in the operation of a rapid coagulation sedimentation basin where no rapid agitation is conducted, an injection rate of inorganic coagulants is increased to such an extent that there is no room left for immediate improvements.
However, the above-described operation heavily depending on the increased injection rate of inorganic coagulants is able to provide results which are substantially satisfactory in terms of sedimentation but raises other technical problems at the stage of filtration and sludge disposal which are subsequent to the sedimentation.
More specifically, with an increase in the injection rate of inorganic coagulants, micro flocks which flow into a filtration basin are made coarse and lower in density as well as flocculates and agglomerates in sedimentation-treated water are increased in residual amount due to an increased volume of the flocks. As a result, there is posed a problem that the filtration basin must be washed more frequently.
Further, regarding the sludge disposal, sludge in itself is developed in an increased amount with an increased amount of inorganic coagulants and the sludge is reduced in concentration and dehydration, thus making the sludge disposal difficult.
A fundamental cause behind the problems on the above-described conventional technologies is that despite the fact that coagulation, sedimentation, filtration and sludge disposal are operated as an integrated system, the operation has been adopted for the system by giving substantially no consideration to optimal filtration or sludge disposal but only emphasizing the formation of flocks having a greater diameter for optimizing the sedimentation, more specifically, with concern for an increased turbidity of sedimentation-treated water in association with destruction of flocks, there has been selected an agglomeration process which is extremely ineffective and lower in agitation intensity and no attention has been given to the realization of high-quality filtration which is a subsequent treatment.
With the above situation taken into account, Patent Document 1 has made such a proposal that there are provided rapid agitation tanks made up with plural stages, a lower limit of agitation intensity is set in a first tank, while an upper limit of agitation intensity is set in a second tank and subsequent tanks, and an inorganic coagulant is injected in a divided manner to each of the rapid agitation tanks, thereby improving the efficiency of particle separation and reducing the concentration of remaining inorganic coagulants (refer to claim 6 and a description related to claim 6).
However, in the above constitution proposed in Patent Document 1, the above effects are obtained insufficiently in that the second and subsequent tanks are limited in agitation intensity more than necessary. Further, an inorganic coagulant is not necessarily adjusted for injection as a whole or criteria for the adjustment are not established. Thus, there is no chance of avoiding such an assessment that the above effects are attained quite insufficiently.
Patent Document 2 has described a process that a hollow contact layer can be arranged to separate micro flocks which are finer in particle size and higher in density. However, the contact layer is needed for washing due to the fact that the layer is clogged more extensively according to retention of the micro flocks, by which the layer is not usable in a sedimentation treatment which is premised on continuous treatment.
More specifically, formation of the micro flocks finer in particle size and higher in density may be partially able to reduce the concentration of remaining inorganic coagulants but unable to satisfy a fundamental technical request for continuous treatment. Thus, there is no chance of avoiding such an assessment that the above process is fatally defective as a coagulation sedimentation process for water to be treated.
Non-Patent Document 4 has described that in place of a conventional coagulation process lower in agitation intensity and higher in injection rate of inorganic coagulants in which flocks will easily leak from a sand layer, it is preferable to adopt a coagulation process higher in agitation intensity and lower in injection rate of inorganic coagulants. However, flocks formed by this coagulation process are made finer in particle size and higher in density, thereby micro flocks remain abundantly in sedimentation-treated water. Nevertheless, since a specific constitution for separating these micro flocks is not indicated at all, there is no chance of avoiding such an assessment that this process is incomplete technically.
Apart from the assessment of individual known technical documents, in coagulation and sedimentation for water to be treated, at a final stage of flocculation or at the final stage thereof or an intermediate stage, a flock-forming inclined plate is provided to contribute to the flocculation of micro flocks. However, the inclined plate is considered to contribute to the flocculation functions only to a supplementary extent. In reality, there has been so far proposed or suggested no technical idea for positively utilizing flocculation functions which are given originally to the flock-forming inclined plate.
Non-Patent Document 4 has actually described the inclined plate but has not disclosed or suggested the above technical idea at all.
[Patent Document 1] Japanese Published Unexamined Patent Application No. 2007-203133
[Patent Document 2] Japanese Published Unexamined Patent Application No. H06-304411
[Non-Patent Document 1] Norihiro Tanbo: Basic Research on Coagulation System in Water Treatment (1) to (4), Journal of Japan Industrial Water Association, No. 361, 363, 365, and 367 (1964. 10, 1964. 12, 1965. 2, 1965. 4)
[Non-Patent Document 2] Committee Report: Capacity and Loadings of Suspended Solids Contact Units, J. AWWA, April 1951
[Non-Patent Document 3] Shogo Thunoda and Katsuyuki Kataoka): Research on Slurry Blanket-Type Rapid Coagulation Sedimentation Plant (2), Effects of Coagulation and Agitation Conditions on Slurry Blanket Layer, Journal of Japan Industrial Water Association, No. 133, pp 39-47, 1969, 10
[Non-Patent Document 4] Design Guide of Water Works, the Japan Water Works Association, issued in 2000.